Nh3-monitoring of an scr catalytic converter

ABSTRACT

The present invention relates to an internal combustion engine ( 1 ) with an SCR catalytic converter ( 11 ) and with a condition monitor ( 10 ) of the NH3 level of the SCR catalytic converter, wherein the condition monitor is connected to a first ( 14 ) and a second detecting module ( 15 ), each of which determines the NH3 level in a different way. In addition, a method for the determination of the NH3 level of an SCR catalytic converter is claimed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT/EP2007/008115 filedSep. 18, 2007.

FIELD OF THE INVENTION

The present invention relates to an internal combustion engine with atleast one SCR catalytic converter and a condition motoring of the SCRcatalytic converter.

BACKGROUND OF THE INVENTION

It is known from the prior art that an SCR catalytic converter isassessed with respect to its functioning. In DE 43 15 278 A1, themonitoring of NH3 storage is discussed in general terms, but there is noconcrete information as to how an NH3 level can be determined. It isdescribed in DE 199 31 007 A1 that during the storage of ammonia,certain physical properties of the SCR catalytic converter change, whichcan be detected metrologically. The applicant's unpublished WO2007/096064 describes a regulation and a change from a lean to astoichiometric operation of a four-stroke engine. In the diesel engine,on the other hand, there is no shift to stoichiometric operation, so theregulation of the engine operation must be done in a different manner ifthe exhaust gas temperature rises sharply. It is known from paragraph 27of EP 17 12 764 A1 that an NH3 balance is used as a method fordetermining an NH3 level of the SCR catalytic converter. These methodshave the following background:

At a low exhaust gas temperature, SCR catalytic converters have a highability to store NH3. In addition, the effectiveness of a catalystincreases with the storage level. An excessively high storage levelshould be avoided, however since a rapid reversal of the storagecapability occurs with increasing temperature, therefore excessive NH3would be emitted to the environment, i.e., an NH3 slip, as it isreferred to below, would occur. For this reason, the storage level mustbe monitored and regulated to a target value.

The problem of the present invention is to enable a reliable and secureoperating mode of an internal combustion engine with an SCR catalyticconverter in which NH3 slip can be securely avoided.

SUMMARY OF THE INVENTION

The problem is solved by an internal combustion engine as well as by amethod disclosed herein. Advantageous configurations and refinementsfollow from the respective subordinate claims.

An internal combustion engine is proposed with at least one SCRcatalytic converter and with at least one condition monitor (10) for theNH3 level of the SCR catalytic converter, wherein the condition monitoris connected to at least one first and one second detecting module thatdetermine the NH3 level in different ways. Preferably a correlation unitis connected to the first and second detecting modules. A refinement hasa stored weighting function by means of which an NH3 slip betweendifferent detections of the NH3 level can be at least partiallycompensated. At least one detecting module preferably comprises a sensorthat is capable of recording a value in relation to the NH3 level.

It is preferred that at least the first and/or the second detectingmodule comprise an integration of a mass flow relative to a supplied andconsumed NH3 mass flow and/or have one or more stored characteristicdiagrams containing a dependency of an NOx conversion on a stored NH3amount in the SCR catalytic converter and/or a physical model of the SCRcatalytic converter that has kinetic approaches to a storage behaviorand/or a characteristic diagram-based determination of a current NH3level of the SCR catalytic converter.

Another configuration provides that the condition monitor be coupled toa load check and/or an SCR temperature check, wherein an NH3 slipavoidance threshold is present, and if it is exceeded, an operating modechangeover of the internal combustion engine is initiated.

It is additionally preferred that the condition monitoring be coupled toan NH3 level regulation. One or more SCR catalytic converters can bepresent. They can be connected in parallel and/or in series. One or moremetering functions for one or more reducing agents can also be present.Correlation can be performed for each individual SCR catalytic converterand/or for several SCR catalytic converters in common.

According to another conception of the invention, a method fordetermining an NH3 level of an SCR catalytic converter for an internalcombustion engine, preferably an internal combustion engine describedabove or below, is proposed, in which a value relevant to a respectiveNH3 level is determined by at least two different determination paths,and they are correlated to deduce a resulting NH3 level. Preferably arespective NH3 level is acquired on the different determination paths,and these are correlated with one another in order to acquire aresulting NH3 level.

A refinement provides that a drift between at least two differentlydetermined values be deduced from the results of the differentdetermination paths.

For example, a diagnostic system can be created with the proposed methodthat uses the different determination paths to check a subsystem fordetermining the NH3 level.

Another configuration provides that a threshold value is set for abeginning of an NH3 slip, and if it is exceeded, the internal combustionengine changes its operating mode. The threshold value can bechangeable, for example, more particularly, adaptable. For example, thethreshold value can be stored in a characteristic diagram or specifiedby a control device.

It is further proposed that at least one of the proposed determinationpaths be used for monitoring an SCR catalytic converter of an internalcombustion engine. Further characteristics and explanations regardingthe proposed internal combustion engine and the method will be describedbelow.

The current NH3 level is determined according to one embodiment in atleast two, preferably several ways, independently of one another. TheNH3 level of the SCR catalytic converter cannot be directly measured.Therefore methods with which the NH3 level can be determined must bedeveloped or used. If NOx sensors are used for this calculation, then itmust be taken into account that these sensors have a certain inaccuracy.Since the storage level is derived from the integral of a difference,e.g., input NH3 amount minus consumed NH3 amount, a considerablyincorrect determination of the storage level results over time from evensmall sensor errors of a few ppm. Another advantage is therefore toachieve a partial compensation or correction of the NOx sensor error byusing different methods for determining the NH3 level.

Moreover, an intervention in the engine control is possible to avoid NH3slip in case of rapidly increasing exhaust gas temperature, so that incase of a temperature increase, higher NOx raw emissions resultsimultaneously, which lead to a faster drawdown of the stored ammonia.For example, a partial compensation of an NOx sensor error as well as acorrection of the sensor signal or a metering can result from multipledeterminations of the storage level.

A first method contains the integration of the mass flows of the meteredNH3 as well as the NH3 consumed for NOx conversion. The stored NH3amount results from the difference of these two components. In thismethod, the metered NH3 amount is determined from the characteristiccurve of the metering system. The converted amount is calculated via theNOx conversion, for example, by using NOx sensors upstream anddownstream of the SCR catalytic converter, or a model for the NOxemissions. These measurement signals or model values are error-prone toa certain extent. Since an integration is involved, the thus-determinedvalue for the NH3 level becomes less accurate over time.

A second method determines the current NH3 level by way ofcharacteristic diagrams that contain the dependence of the NOxconversion on the stored NH3 amount. This dependence is determined forthe SCR catalytic converter by prior experiment. The final value of theNH3 level is determined by means of a weighting of the partial resultsfrom the methods used. The weighting can be a function of the variousinput parameters, for example, the catalytic converter temperature orthe exhaust gas mass flow. Alternatively, the arithmetic mean can betaken.

The behavior of the first and the second methods will be described inmore detail below. The first method takes into account the completemetered mass flow of the reducing agent. The fact that the reducingagent must possibly first be converted to NH3 via intermediate stepssuch as thermolysis or hydrolysis is ignored. In addition, part of thereducing agent may not be available at the SCR catalytic converter atall, due to unequal distribution or the formation of deposits. For thisreason, the NH3 level determined by the first method is fundamentallyhigher than the actual NH3 level available for NOx conversion. Incontrast, the second method directly monitors whether an NH3 levelsufficient for the desired NOx conversion is available. If the NOxconversion is lower than desired, then the calculated level will bereduced and more reducing agent will be metered in. However, anexclusive use of the second method has the risk that the NOx conversioncalculated by cross-sensitive NOx sensors will continue to decline incase of an NH3 slip, which would result in a further increase of thereducing agent metering and thus a higher and higher NH3 slip. This canbe prevented by the simultaneous use of the first method, which includesthe absolute metered amount and thus prevents a larger and largerincrease of the metered amount.

In principle, the two methods exhibit the opposite behavior in case ofan erroneous signal of the NOx sensors. If two NOx sensors are used forthe regulation, for example, one sensor upstream and one downstream ofthe SCR catalytic converter, and if these two sensors have the sameerror, this will have no effect on the regulation since only differencesignals are used. In the case of different sensor errors, on the otherhand, an erroneous determination of the NH3 level results, insofar asonly one of the above-mentioned methods is used. A combination of thefirst and the second methods, on the other hand, allows a partialcompensation of the sensor error. If, for example, the downstream NOxsensor indicates an excessively high value caused by a sensor drift oran NH3 slip, then an excessively low NOx conversion is calculated. AnNH3 level that is higher than the actual level results in the firstmethod due to the integration of the difference between the metered andthe converted NH3 amounts. On the other hand, the second methoddetermines a lower level than actually exists. An overall more plausibleNH3 level is determined from the averaging of these individual values,so that the regulation remains stable even in case of a sensor error.

An excessively large deviation of the two determined levels can also beused for adapting the NOx sensor or the metering. If such a deviation isrecognized over an applicable period of time, then the metering is firstreduced in order to check whether there is an NH3 slip. If the deviationis not thereby reduced, then a sensor drift can be deduced and acorrection of the sensor signal can be performed. If, on the other hand,an additional ammonia sensor is used downstream of the SCR catalyticconverter, then an NH3 slip can be directly measured and the reductionof the metered amount to check for an NH3 slip can be omitted.

Alongside the above-described first and second methods, additionalapproaches with which the NH3 level can be determined are possible, andtheir partial results can flow into the weighting for determining theoverall NH3 level.

A third method provides a physical model of the SCR catalytic converterthat models the storage behavior by means of kinetic approaches, basedon material data specific to the catalytic converter, such as celldensity, volume, specific surface, coating material, etc. It can also beused for resetting the monitored NH3 storage by setting the NH3 level tozero at a high exhaust gas temperature after the lapse of an applicabletime. Such a model can be parameterized by comparison to laboratorystudies of an identical SCR catalytic converter.

A fourth method is a characteristic diagram-based determination of thecurrent NH3 level. In this case, the NH3 level is determined as afunction of the feed ratio, for example, the metered NH3concentration/NOx concentration upstream of the SCR catalytic converter,and boundary values determining the NOx conversion, e.g., temperature,spatial velocity, NO2/NOx ratio upstream of the SCR catalytic converter,etc., as well as the time constant for the storage process. Based onthese values, the NH3 level can be determined by integration of themetered NH3 and NOx amounts.

In addition, a metrological determination of the NH3 level can beperformed by utilizing the fact that physical properties of the SCRcatalytic converter change when NH3 is stored. These connections aredescribed in the above-mentioned patent DE 199 31 007 A1, which isincorporated in full by reference in this regard within the scope of thepresent disclosure. A metrological method for determining the NH3 level,which, in addition to the formation of a partial result, can flow intothe determination of the overall level, is already described in DE 19931 007 A1. The shift from lean to lambda-1 operation is described in WO2007/096064. But even in a purely lean operation, a sharp increase inthe load can lead to a rise of the exhaust gas temperature and thus areduced NH3 storage capability, so that an intervention in the engineoperation is necessary in order to be able to reduce the storage levelon time. For the possibility of how the change of operating mode can beperformed, WO 2007/096064 is incorporated in full by reference withinthe scope of the present disclosure. In relation to a possibleconfiguration of a balancing, EP 1 712 764 A1 is incorporated byreference.

For example, a rapid rise of the SCR catalytic converter temperature canoccur in case of a sharp increase in the load. This has the effect that,even with metering deactivated, the amount already stored in the SCRcatalytic converter can no longer be completely reacted in the form ofNOx conversion, but can escape into the environment as NH3 slip. Thiscan be countered by switching the engine into a different operating modewith higher raw NOx emissions and possibly simultaneously lower fuelconsumption due, for example, to a reduced exhaust gas return rate or anadvanced beginning of injection.

NOx sensors have a maximum possible accuracy that may not be sufficientfor exact regulation of the metering, and moreover, they reactcross-sensitively to ammonia. Therefore it is currently necessary to usemodel-based regulation systems that are difficult to supply with data,or the metering regulation is deliberately set up such that the maximumpossible NOx efficiency is not used, in favor of avoiding NH3 slip. Theadvantage of the technical teaching described here is that at leastpartial compensation of measurement errors becomes possible by usingseveral different methods for determining the amount of NH3 stored inthe SCR catalytic converter, wherein the influence of sensor deviationsfor two methods is opposite, so that a compensation of the error isrealized, or recognition of NH3 slip or a sensor error becomes possible.An adaptation of this sensor or the metering is thereby possible.

In case of a sharp temperature rise of the SCR catalytic converter, itis possible, according to another conception of the invention, alsoindependent, to avoid a slip of the ammonia stored at a lowertemperature by adjusting the engine operating mode in such a way thatthe raw NOx emissions are elevated and the increased NH3 conversionnecessary for the reduction of these nitrogen oxides lowers the NH3level sufficiently quickly. Such an engine operating mode can also leadat the same time to a lower fuel consumption.

According to an additional conception of the invention, the NH3 levelcan also be determined in other ways in addition to the methodsdescribed above. If more than two methods are used, the effort to supplydata and the complexity of obtaining plausible information alsoincrease. A weighting of the individual components can be introduced forthe averaging to determine the overall NH3 level. This can also bedesigned to be temperature-dependent. For example, the characteristicdiagram-based level can be assigned a higher weight in this manner atlow temperatures, while the level determined from the balance can beassigned a higher influence at high temperatures.

Various advantages of the invention, each of which can also beindividually pursued further as an invention, independently of theothers, will be presented below:

-   -   stand-alone determination of the current NH3 storage level in        several ways with subsequent weighting and formation of an        overall value;    -   use of a characteristic diagram that takes into account the        dependence of the NOx conversion on the level;    -   compensation of measurement errors occurring from sensor errors        or NH3 slip by contrary influence of this measurement error on        the methods used;    -   checking of the deviations between the results of the methods        employed, and adaptation of the metering in case of recognized        NH3 slip or adaptation of the NOx sensor value in case of a        recognized sensor error;    -   inclusion of additional level determination and/or metrological        detection of the level;    -   shift of the engine operating mode to increase the raw NOx        emission in case of a rise in the exhaust gas temperature in        order to rapidly draw down the stored ammonia to avoid NH3 slip;

A particularly preferred application of the invention, which can also bepursued independently of the others, results as follows, for example:

-   -   regulation of the NH3 level, particularly at low exhaust gas        temperatures in the motor vehicle, in order to achieve high NOx        conversion;    -   avoidance of a heating strategy of the SCR catalytic converter        in case an optimized regulation leads to conformance with the        NOx limit values already at low temperature, thereby avoiding        extra fuel consumption;    -   avoidance of an extra NH3 trap catalytic converter downstream of        the SCR catalytic converter, for example, if the regulation is        limited to an NH3 breakthrough at a minimal level; thereby        saving an additional component and thus costs and installation        space as well as additional calibration effort, particularly in        case of ODB;    -   change of the engine operating mode toward higher raw NOx        emissions with simultaneously reduced fuel consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below on the basis of illustrationexamples. The details and features evident from these illustrations arenot to be interpreted as limiting, however. Rather, they are to beunderstood only as one of several possible implementations orpossibilities. Moreover, characteristics evident from the individualfigures can be linked with other characteristics from other figures orfrom the above general description to form additional configurations. Indetail:

FIG. 1 shows a schematic representation example of the arrangement of aninternal combustion engine, an SCR catalytic converter and additionalcomponents;

FIG. 2 shows a representation example of the dependence of an ammoniastorage capability versus the temperature of an SCR catalytic converter,

FIG. 3 shows a representation of an NOx conversion rate as well as anammonia slip relative to an ammonia level of an SCR catalytic converter,

FIG. 4 shows a schematic representation of the determination of an NH3level in an SCR catalytic converter in various ways and its furtherprocessing,

FIG. 5 shows a compensation of at least two different determinationpaths of an NH3 level for acquiring a level arising therefrom,

FIG. 6 shows a representation example of regulation of an NH3 level bymeans of a regulator integrated with the monitor, and

FIG. 7 shows a contrast of various operating modes of the internalcombustion engine, wherein an NH3 slip appears in the upper area of FIG.7 if there is no change of the operating mode, and the prevention of anNH3 slip by changing the operating mode is illustrated in the lower areaof FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a possibility of arranging various components of the systemin a representation example. This arrangement is not to be interpretedas restrictive, however. Rather, various components can also be arrangedat different places. An internal combustion engine 1 can be seen inFIG. 1. It is connected to an exhaust gas system 2. A flow direction ofan exhaust gas is indicated by the arrows 3. An oxidation catalyticconverter 4 is arranged downstream of internal combustion engine 1, forexample. In place of the oxidation catalytic converter 4, there couldalso be an exhaust gas return directly into internal combustion engine 1and/or an exhaust gas turbine of an exhaust gas turbocharger. A firstNOx sensor 5 is arranged downstream of oxidation catalytic converter 4,for example. The former is preferably arranged upstream of an inlet of areducing agent supply line 6 in exhaust gas system 2. Reducing agentsupply line 6 has a valve 7, for example. A metered amount of a reducingagent can be supplied in a controlled manner or regulated in a targetedmanner by means of this valve, for example, an injector. Valve 7 isconnected for this purpose via a data line 8 to a control device 9, forexample, an engine control device. A condition monitor 10 for an SCRcatalytic converter 11 is preferably contained in control device 9.Control monitor 10 can also be housed, however, in a separate control orregulation device that is connected to control device 9. For the sake ofexample, at least one temperature sensor is assigned to SCR catalyticconverter 11. The temperature sensor 12 is upstream of SCR catalyticconverter 11 according to this configuration. It can also be integratedinto the SCR catalytic converter, however, or situated downstream of it.One or more temperature sensors 12 can also be provided at various ofthese sites, in order to allow temperature monitoring of the exhaust gasstream and/or SCR catalytic converter 11. A second NOx sensor 13 isarranged downstream of SCR catalytic converter 11. The NOx sensors canalso be arranged in a different manner, not limited to the arrangementpresented here. In addition to condition monitor 10, the control device9 according to the configuration presented here additionally implementsa first detecting module 14, a second detecting module 15, a correlationunit 16 and a weighting function 17. These individual components canpreferably be situated in the same control device but can also bepresent in different units physically separated from one another. Herethey are equipped with a suitable signal transmission path such as a bussystem. In order to obtain one or more values necessary for therespective calculation method stored in detecting modules 14, 15, theycan be connected, for example, to one or more sensors. In particular,the result with respect to an NH3 level acquired from first detectingmodule 14 and from second detecting module 15 can be correlated viacorrelation unit 16. For example, it is provided that the acquiredresults be adapted via a weighting function 17, so that the overall endresult is an NH3 level with which a regulation can be operated. Aregulation of the NH3 level is preferably performed by means of an addedregulator, which is likewise preferably integrated into control device9. A load monitor 18 is also provided. The load can be monitored, forexample, via a pedal position as illustrated. However, the torque or therotational speed of internal combustion engine 1 can also be monitoredfor this purpose. In addition to these illustrated components,components such as sensors, monitoring units and/or additional catalyticconverters can be provided, however, they are not shown here in detailfor reasons of simplification.

With the internal combustion engine 1 presented here, there is thepossibility that several, preferably two, different determination pathsare used to be able to determine the level value of SCR catalyticconverter 11 more accurately. The first two determination pathspresented below are particularly suitable, since the errors in thedetermination of the level compensate one another at least in part ifone determines the level from a weighted average, for example.

The first determination path is to prepare an ammonia balance from theamount of supplied ammonia, which is known from the cycle time of themetering valve, and from the difference of the NOx values upstream anddownstream of the SCR catalytic converter 11 as measured by two NOxsensors. Instead of the NOx sensor upstream of SCR catalytic converter11, a characteristic diagram or a model of the NOx emissions of internalcombustion engine 1 could alternatively be used. Under the largelysatisfied condition that NOx is not stored to a great extent in SCRcatalytic converter 11, the amount of consumed ammonia can be determinedfrom the measured NOx difference. The remainder of the ammonia mustconsequently be stored in SCR catalytic converter 11 or, in the case ofa negative balance, has been depleted. The instantaneous level isobtained by integration of the respective stored amounts. This balancedoes not take into account an ammonia slip, which should of course beavoided with proper handling of the process. In case of a slip, thecross-sensitivity of the downstream NOx sensor to ammonia also comesinto play. This sensor upstream of the catalytic converter is notsubjected to ammonia, since it is situated upstream of the injectionpoint for ammonia.

The second determination path likewise provides the measurement of thesupplied ammonia and the NOx values upstream and downstream of the SCRcatalytic converter. The storage level is not determined in this case byintegration from the NOx conversion measured as in the firstdetermination path; instead, the level is determined directly as afunction of NOx conversion by way of a characteristic diagram “Ammonialevel vs. NOx Conversion.” The NOx conversion is a function of theammonia availability, in addition to the temperature, the NO₂/NO_(x)ratio, the exhaust gas mass flow and other boundary conditions, and thusalso of the ammonia level. This dependence is used for determining thelevel. The advantage is that this method does without integration andthus does not become more and more imprecise over time like the firstdetermination path. According to one configuration, this characteristicdiagram method also does not take into account an ammonia slip or thecross-sensitivity of the second NOx sensor to ammonia.

The crucial advantage of the combination of the two determination pathsis that errors of measurement due both to ammonia slip and sensor errorscan be recognized and partially compensated by averaging the acquiredlevel values. In the two determination paths, the effect of ammonia slipand sensor errors enter the determination paths in opposite directions.If, for example, ammonia slip occurs, then the second NOx sensor, whichis downstream of the SCR catalytic converter, will always measure anexcessively high NOx value due to the cross-sensitivity to ammonia. Inthe first determination path, an excessively low NOx and ammoniaconversion will be determined and therefore the determined ammonia levelwill be too high. In the second determination path, on the other hand,an excessively low ammonia supply will be diagnosed from the low NOxconversion rate via the characteristic diagram and thus the ammonialevel will be too low. A plausible level value for the ammonia can beachieved by appropriately weighted averaging. The analysis is completelyanalogous for sensor errors, for example. They also behave in oppositeways.

It may additionally be pointed out that each determination path canitself have a correction factor or some other value with which adeviation, a drift and/or some other change, can be compensated. Thiscan also be provided for the determination paths proposed here and theirrespective linking with one another.

A diagnosis method for a level drift can also be performed by at leasttwo different determination paths. For this purpose, for example, thesupply of ammonia is reduced under otherwise fixed operating conditions;if the levels measured by both methods drift closer to one another as areaction in the direction described, then an ammonia slip should bediagnosed as a cause for the drift, i.e., the level lies at or above thelimit for ammonia slip. If the level values drift further apart, thenthere is too little ammonia in storage, caused, for example, by a sensorerror. These errors can then be compensated by correcting the sensorsignal or the metering. An analogous determination can also be made withan increased supply of ammonia. This diagnosis can be used forregulation, for a limit value check or as a plausibility criterion. Inthis way, for example, the state of the regulation or a threshold valuecan also be monitored, possibly with a subsequent adjustment byadaptation.

For a third determination path, for example, only the input temperaturesand the ammonia and NOx quantities are required in addition to alreadyavailable characteristic parameters of the SCR catalytic converter suchas cell density, material properties and so on, since a physical modelis capable of calculating the output values, including the storagelevel, on its own. This method can be used according to an additionalconception of the invention as an additional independent method,particularly for checking the plausibility of the combination of thefirst and second determination paths, as well as being used as anindividual measuring method.

A fourth determination path treats the SCR catalytic converter 11 as afirst-order regulation timing element with respect to the storage. Forthis purpose, the time constants or the behavior over time of theammonia storage is input into a characteristic diagram as a function ofthe temperature and the level. The ammonia level can thus be determinedat any time from the supply of ammonia and NOx, measured according tothe third determination path, for example. The timing element representsan integration. Here, for example, the detailed physical model of thethird determination path is replaced by a black box with PT1 behavior inthe storage process and DT1 behavior in the emptying of the storagelevel.

FIG. 2 shows a correlation between an ammonia storage capability,represented on the Y-axis, and a temperature of an SCR catalyticconverter, represented on the X-axis. At a low exhaust gas temperature,SCR catalytic converters have a high ability to store NH3 . Moreover, anefficiency of an SCR catalytic converter increases with a storage level.An excessively high storage level should be avoided, however, since arapid reversal of the storage capability occurs with increasingtemperature, as shown, and therefore excessive NH3 would be emitted tothe environment. This would result in a so-called NH3 slip. For thisreason, the NH3 level of an SCR catalytic converter is monitored and,based on a knowledge of the correlation seen in FIG. 2 specifically foran SCR catalytic converter, a target value is preferably regulated, butis at least initially controlled. In addition, this correlation is usedto be able to define one or more different threshold values, forinstance, for an NH3 slip.

FIG. 3 shows, in a simplified representation, a correlation between anammonia storage level in an SCR catalytic converter, represented on theY-axis [sic; X-axis], and an NOx conversion or an NH3 slip, representedon the Y-axis. The higher the NH3 level of the SCR catalytic converter,the greater the possibility that an NH3 slip will occur. The amount thatcan escape into the environment in case of such an NH3 slip also becomeslarger with an increasing NH3 level. Furthermore, since the storagecapacity decreases with increasing temperature but NOx emissions alsoincrease with rising temperature, whereas, on the other hand, theefficiency of an SCR catalytic converter in converting NOx increaseswith increasing NH3 level, it has been found, according to anotherconception, which can also be further developed, that it is advantageousto provide an intervention in a controller of the internal combustionengine so that higher raw NOx emissions occur in case of a rise intemperature, which lead to a faster drawdown of the stored ammonia.

FIG. 4 shows a configuration of a possible process sequence in anexample schematic diagram. Different ways of determining the NH3 levelare used here, briefly designated as NH3 balance, characteristic diagramand kinetics model. They can be supplemented by additional types ofdetermination, indicated by the empty box. They are each provided with aweighting factor, indicated by the weighting function 17. A temperature,a water flow or some other parameter can serve as input parameters for aweighting. From the totality, an NH3 level is determined, which ispreferably a component of a regulation of the NH3 level of the SCRcatalytic converter.

FIG. 5 shows a configuration example of the invention, in which acompensation is used that is based, for instance, on types of NH3 leveldetermination tending to go in different directions. Thus, thedetermination by way of an NH3 balance tends to move in a differentdirection than the determination of the NH3 level by a type ofcharacteristic diagram calculation. These two correlated methods atleast reduce the otherwise existing deviation error and, in particular,can even cancel one another out with a suitable correlation. In case ofexcessive deviations, this additionally allows a check of whether one ormore of the recorded values is possibly erroneous. In this way, anoperational message can be provided as to whether there is an errorwith, for example, a sensor, a measurement unit, a correlation unit, adetecting module or possibly an SCR catalytic converter. Thepossibilities for compensation can also be different. This can takeplace, for example, by weighted averaging. The respectively determinedNH3 level values in particular can be at least partially compensated byaveraging.

FIG. 6 shows a configuration example of a regulation scheme fordetermining an NH3 level of an SCR catalytic converter. The NH3 level isindicated as NH_(3Stor) _(—) _(act). This represents the result of thissection of the regulation method. According to this representation, anNH3 level is determined in two ways. First, a first NH3 level isdetermined via an NH3 balance. This value NH_(3Stor) _(—) _(Balance)enters into the process just like an NH3 level determined by means of acharacteristic diagram that takes into account a dependence of an NOxconversion on the NH3 level. This value is specified as a partialresult, NH_(3Stor) _(—) _(charact. diag). With regard to thedetermination by way of a balancing, for example, an integration of adifference of the metered and converted NH3 mass flow is performed. Thedetermination by means of the characteristic diagram, on the other hand,contains test results for a dependence between the NH3 level and the NOxconversion. An NH3 level can thereby be associated with the measured NOxconversion. This result is corrected by an additional characteristicdiagram which takes into account the fact that the NOx conversion is afunction of additional boundary conditions such as the NO₂/NO_(x)conversion, the NO₂/NO_(x) ratio, the spatial velocity, etc., alongsidethe temperature and the NH3 level. The two partial results are combined,weighted via a temperature-dependent characteristic curve, into theoverall result. The values emerging from FIG. 6 are composed as follows:

-   NH_(3 MET): metered NH₃ (signal from the metering device)-   NH_(3CONV): converted NH₃-   NO_(XV): NO_(x) concentration upstream of SCR catalytic converter    (signal from characteristic diagram, calculation or sensor)-   NO_(XV)[sic; NO_(XN)]: NO_(x) concentration downstream of SCR    catalytic converter (NOx sensor)-   TSCR: SCR catalytic converter temperature-   NH_(3 ST) _(—) _(Balance): stored NH₃ from balance-   NH_(3ST) _(—) _(charact. diag.): stored NH₃ from characteristic    diagram-   NH_(3ST) _(—) _(act): stored NH₃-   ETA_(SCR): efficiency of the SCR catalytic converter-   NO₂/NO_(x): ratio of NO₂ to NO_(x) concentration upstream of SCR    catalytic converter-   RG: spatial velocity

FIG. 7 shows in an upper representation that a rapid rise of the SCRcatalytic converter temperature can occur in the case of a sharpincrease in the load. In comparison to this, an increased load at thesame time is indicated by a dotted line in the lower representation. Dueto the elevated temperature, a higher formation of NO_(x) occurs, and atthe same time, there is a decrease of the storage capability of NH₃ inthe SCR catalytic converter. This is indicated by the dashed curve,which indicates the maximum NH₃ that can be stored, while the currentlystored NH₃ is indicated by the dot-dash line. Due to the increased loadand the temperature increase generated thereby, there can be the effectthat, even with metering deactivated, the amount already stored in theSCR catalytic converter can no longer be completely reacted in the formof NO_(x) conversion. This causes the ammonia to escape into theenvironment when the currently stored NH₃ value and the maximum storableNH₃ value intersect. This is illustrated by the NH₃ slip that is shown.The operating mode of the internal combustion engine shown thereunderindicates that the temperature of the SCR catalytic conversion alsorises in case of an increased load. Therefore the maximum storable NH₃also declines here. By changing the operating mode of the internalcombustion engine, however, a higher NH₃ value can be made available. Ifin addition to a higher raw NOx emission, a lower fuel consumption isachieved, for example, by means of a reduced exhaust gas return rate oran advanced beginning of injection, the slip formation can be countered.As shown, the currently stored NH₃ level decreases in such a manner thatit remains below the maximum storable NH₃ limit value. As shown above,the maximum storable NH₃ value can also be used as a limit value inorder to check to what extent the regulation and, in particular, a loadchangeover actually is functioning. For example, monitoring can beassured in this case by a sensor recording of a possible NH₃ slip.

1. An internal combustion engine comprising: at least one SCR catalyticconverter; and at least one condition monitor of SCR catalytic converterfor its NH3 level, wherein the condition monitor is connected to atleast one first and one second detecting module that determine the NH3level in different manners.
 2. The internal combustion engine accordingto claim 1, further comprising a correlation unit connected to the firstand second detecting module.
 3. The internal combustion engine accordingto claim 1, wherein a stored weighting function by means of which an NH3slip can be at least partially compensated between different detectionsof the NH3 level.
 4. The internal combustion engine according to claim1, wherein at least one of the first and second detecting moduleincludes a sensor that is capable of recording a value in relation tothe NH3 level.
 5. The internal combustion engine according to claim 1,wherein at least the first and/or the second detecting module includesan integration of a mass flow relative to a supplied and consumed NH3mass flow and/or has one or more stored characteristic diagramscontaining a dependence of an NOx conversion on a stored NH3 amount inSCR catalytic converter and/or a physical model of SCR catalyticconverter that has kinetic approaches to a storage behavior and/or acharacteristic diagram-based determination of a current NH3 level of SCRcatalytic converter.
 6. The internal combustion engine according toclaim 1, wherein the condition monitor is coupled to a load check and/oran SCR temperature check, wherein an NH3 slip avoidance threshold ispresent, and if the threshold is exceeded, an operating mode changeoverof the internal combustion engine is initiated.
 7. The internalcombustion engine according to claim 1, wherein the condition monitor iscoupled to an NH3 level regulation.
 8. A method for determining an NH3level of an SCR catalytic converter for an internal combustion engineaccording to claim 1, said method comprising the steps of: determining avalue relevant to a respective NH3 level by at least two differentdetermination paths, and wherein the at least two differentdetermination paths are correlated to deduce a resulting NH3 level. 9.The method according to claim 8, wherein a respective NH3 level isacquired by different determination paths, and wherein the differentdetermination paths are correlated with one another in order to arriveat a resulting NH3 level.
 10. The method according to claim 8, wherein adrift between at least two differently determined values is deduced fromthe results of different determination paths.
 11. The method accordingto claim 8, wherein a diagnostic system can be created that usesdifferent determination paths to check a subsystem for determining theNH3 level.
 12. The method according to claim 8, wherein a thresholdvalue is set for a beginning of an NH3 slip, and if the threshold isexceeded, the internal combustion engine changes its operating mode. 13.Application of at least one of the determination paths according toclaim 8 for monitoring an SCR catalytic converter of an internalcombustion engine.
 14. An internal combustion engine comprising: a SCRcatalytic converter; a condition monitor for monitoring a NH3 level ofsaid SCR catalytic converter; a first detecting module connected to saidcondition monitor; and a second detecting module connected to saidcondition monitor; wherein said first detecting module determines saidNH3 level of said SCR catalytic converter in a first manner, and whereinsaid second detecting module determines said NH3 level of said catalyticconverter in a second manner different from said first manner.